Monitoring systems are used for determining the presence or concentration of analytes in body fluids, such as glucose, cholesterol, alcohol, and hemoglobin in blood, interstitial fluid, or chemical substances in saliva. These monitoring systems require frequent use of test sensors, which are commonly used to test harvested blood or any other suitable liquid sample.
Typically, a user will deposit a test sample of the biological liquid on a sample receiving area or pad either in fluid communication with the test sensor, or forming a portion of the test sensor. The biological liquid sample is permitted to wick along the test sensor to a predefined testing area that includes a reagent capable of a readable change when contacted by a predetermined constituent in the test sample.
Test sensors commonly include at least a pair of electrodes, including a working electrode and a counter electrode. Test sensors also commonly include a third electrode, a trigger electrode. The trigger electrode is electrically in parallel with the counter electrode but, when clean, can supply a small current pulse, which can be used to start the meter timing sequence and detect whether the sensor is inadequately filled with fluid sample.
The test sensor also includes a dry reagent in contact with the working electrode and counter electrode, and a capillary flow channel extending from an inlet opening to the working and counter electrodes. The reagents typically include an enzyme that is capable of oxidizing the glucose in the sample, such as glucose oxidase and one or more mediators adapted to reoxidize the reduced enzyme resulting from oxidation of the glucose, thereby forming a reduced mediator. The test sensor is inserted into a meter so that the working and counter electrodes are electrically connected to the components within the meter. After the test sensor is inserted in the meter, a sample of a bodily fluid, such as blood, is introduced into the capillary flow channel and contacts the trigger electrode, if present, working electrode, counter electrode and reagent, whereupon the components within the meter apply one or more electrical voltages between the working and counter electrodes. These electrodes transmit the electrical signals generated by the test sensor to a processor in the meter and the electrical current passing between the electrodes is measured. The reduced mediator is oxidized at the working electrode, thereby producing a measurable current which is related to the amount of reduced mediator present at the working electrode, and therefore related to the concentration of glucose in the fluid. The measured current typically begins at a high value and then declines and approaches a constant value. For example, the current measured at a predetermined time during application of a voltage may be used to determine the glucose content of the sample. The processor then analyzes these signals and displays the results (e.g., analyte concentration level) to the user via a display device.
Accurate test results are dependent on a variety of factors, including providing an appropriate amount of fluid sample on the test sensors and properly functioning electrodes. To help provide more accurate test results, test sensors commonly include a third electrode, sometimes referred to as a trigger electrode. The trigger electrode is electrically in parallel with the counter electrode, but is capable of supplying a small current pulse. This pulse can be used to start the meter timing sequence to begin testing at a point in time in which the sensor is adequately filled with fluid sample. Similarly, the pulse can be used to determine whether there is an inadequate amount of fluid sample on the sensor and testing should not begin. The trigger electrode can therefore serve as an indicator as to when it is appropriate for testing of the fluid sample to begin.
Trigger electrodes are commonly positioned upstream of both the working and counter electrode toward an outermost edge of the test sensor. Because of its position at the edge of the test sensor, trigger electrodes are often contaminated by smoke and other byproducts created during manufacture of the test sensor. During test sensor manufacture, the final shape of the test sensor must be cut out from a laminated multilayer structure. Laser cutting is one method of cutting out the test sensor that provides accurate and reliable results. However, as the front edges of the test sensor are cut out, smoke from the laser contaminates the trigger electrode. In addition, the use of catalytic noble metals, such as gold, platinum, and palladium, can result in the adsorption of airborne contaminants that can also foul the surface and make it less reactive and able to function as a trigger electrode.
Contaminants on the trigger electrode are one common cause of trigger electrode malfunction and inaccurate test results. When the trigger electrode does not function properly, the test may fail to start at the appropriate time. Conversely, the trigger electrode may fail to indicate that there is an insufficient amount of fluid sample on the test sensor and testing should be delayed.
In view of the shortcoming associated with contaminated trigger electrodes, it is desirable to provide a test sensor and a method of making a test sensor that can minimize contamination of the trigger electrode during test sensor manufacture.